1. Field of the Invention
The present invention relates to a variable gear ratio type rack bar and a steering apparatus having the same, and more particularly to a variable gear ratio type rack bar, and a steering apparatus for a vehicle including such a variable gear ratio type rack bar, the rack bar having two or more rack-formed parts with different inter-tooth spaces, wherein the rack-formed parts make it needless to fabricate a separate mold for processing them, the rack-formed parts can be formed by using various processing methods, and the rack-formed parts make it possible to obtain precise tooth shapes through one step processing.
2. Description of the Prior Art
FIG. 1 schematically shows the construction of a conventional rack-and-pinion type steering apparatus for a vehicle.
As shown in the drawing, the conventional rack-and-pinion type steering apparatus includes: a steering wheel 100 positioned at a driver seat side; a steering shaft 105 connected to the steering wheel; a steering column 103 adapted to allow the steering shaft to be fastened to a vehicle body; a gear box 130 including a rack 110 and a pinion 120 for converting torsional force input from the steering shaft 105 to rectilinear movement; a rack bar 140 including a rack bar body 115 on which the rack 110 is formed; and a pair of tie rods 150 integrally formed with balls of inner ball joints 135, respectively, the inner ball joints being provided at the opposite ends of the rack bar 140, respectively, and the tie rods 150 being connected with outer ball joints 155, respectively, so that power is transmitted to knuckles 159 so as to steer tires 158.
In general, a steering apparatus for a vehicle is an apparatus for allowing a driver to change the running direction of a vehicle in accordance with the driver's intention while the vehicle is being run, wherein as shown in FIG. 1, the steering apparatus includes: a steering wheel capable of being rotated by the driver; a steering shaft having an end coupled to the steering wheel to be rotated together with the steering wheel; a pinion provided at the lower end of the steering shaft and positioned within a gear box, the pinion being rotated together with the steering shaft; and a rack bar, on which a rack is formed to be engaged with the pinion, the rack bar conducting rectilinear movement in accordance with the rotation of the pinion.
With the steering apparatus configured as described above, as the steering angle of the steering wheel is increased, torque applied to the pinion is increased due to the tires' friction and the geometry of the vehicle, wherein in order to reduce the torque applied to the pinion, a hydraulic power steering system increases hydraulic pressure, and an electric power steering system increases the power of a motor.
In general, the angle for manipulating the steering wheel for a vehicle is typically limited within 90° while the vehicle is running on a road, and the steering wheel may be manipulated over 90° only when parking or U-turning the vehicle.
However, since the gear ratio of the rack and pinion of such a steering apparatus is determined to be constant, inconvenience to necessarily manipulate the steering wheel to a large extent is accompanied at the time of parking or U-turning.
In addition, since in increasing hydraulic pressure or motor power for reducing torque applied to a pinion, there was a limitation as well as a problem of inefficiency, variable gear ratio (VGR) systems have been developed in order to solve these problems.
Here, a VGR system means a system in which the gear ratio of a pinion, to which steering wheel manipulating force is input, and a rack cooperating with the pinion is variable in such a manner that the input angle at the opposite side parts of the rack differs from that at the central part of the rack, so that when the manipulating extent of the steering wheel is large, the moving distance of the rack is increased, thereby reducing the required manipulating extent of the steering wheel at the time of parking or U-turning, which helps a driver to conveniently drive the vehicle.
The above-mentioned VGR system is a system having a rack, the shape of which is modified in the following manner: inter-tooth spaces of the teeth of the rack are increased as approaching to the opposite ends from the central part of the rack, so that as the steering angle of the steering wheel is increased, the moving distance of the rack is also increased, thereby reducing the torque applied to the pinion.
Such a conventional VGR steering system has problems in that when forming rack-formed parts on a rack bar body using a bobbing machine or a broaching machine, it is impossible to process the rack-formed parts with different inter-teeth spaces at once, and even if the rack-formed parts are processed through plural separated steps, the processing error of the rack-formed parts become severe, which in turn causes the precision of the rack to be deteriorated.
In addition, if a forging process is employed so as to process such rack-formed parts at once, there are problems in that the precision is deteriorated in terms of the shape and size of the rack, which in turn causes a driver's steering feeling to be deteriorated when the driver manipulates the steering wheel.
Furthermore, if a precise forging process is employed for securing the precision in terms of the shape and size of the rack, there are problems in that the length of production time is substantially increased, and the manufacturing cost is very high.